From the Archives: How to accurately measure snowfall

Want to know how much snow fell in your yard? Don’t just poke a yardstick through it.

Who says? Lenny Llewellyn of Mattituck, that’s who. And as a cooperative observer of things meteorological for the National Weather Service, he insists on doing things right — a point he made in early 2011 after The Suffolk Times ran a photo of a yardstick in a snowbank. Last year’s unseasonably warm winter offered no opportunity to set us straight.

With the recent snowfall, Mr. Llewellyn established 10 separate measuring spots in a curving trail from his front to back yards. At each spot, a plywood square, 24 inches on a side, sits on the snow. They’re not heavy enough to compress the snow and they’re painted white to reflect sunlight and prevent melting, but their purpose is to create an even, solid surface. He does employ a yardstick, but only within the square.

When snow depth at all 10 spots is measured to within a 10th of an inch, the average provides a measurement as close to accurate as you can get, Mr. Llewellyn said.

He determined that the snow earlier this week measured 3.3 inches — on his property, anyway.

At times, he’ll take it a step further and measure the snow’s water content. The rule of thumb is one inch of rain equals 10 inches of snow, but there are wide variations in that ratio.

“A coastal storm will not only pull down cold air from the north, it will draw moisture from the south and the snow will be very, very heavy because of the water content,” he said. “But an Alberta Clipper, which comes down from Canada, is dry because it has no opportunity to pick up water.”

To determine the water content, he pulls the eight-inch aluminum tube from his rain measuring device and takes a core sample. He heats the snow and collects the water in a 2 1/2-inch plastic tube.

With a chance of more snow over the next several days, he’s ready to head outside again. But how, exactly, do you measure new snow on top of old snow?

The most accurate method involves clearing away the accumulation from the test spots, said Mr. Llewellyn. But if that’s not possible, he places a Plexiglas sheet on top of his plywood squares and measures up from them, averaging out the counts, of course.

Mr. Llewellyn has monitored the weather closely since 1988 and began sharing his reports with the National Weather Service in 2008. He said his wife, Marjory, “is kind of a weather bug herself. Her grandfather had a barometer and thermometer by the back door and he checked it every day at 6 a.m. until the day he died.”

As for the rest of this winter, Mr. Llewellyn says expect the worst.

“I have a funny feeling that we’re going to make up for what we lost last year,” he said. “I think this winter is going to be a roller coaster” with warm periods but then “Mother Nature will do a complete 360 and do a number on us.”

February is often the coldest month of the year and while March is a transitional time, Mr. Llewellyn says, “I’ve seen some pretty healthy snowstorms in March, so stay tuned.”